1. Field of the Invention
The present invention relates to a process for preparing wholly aromatic polyesters involving the in situ esterification of aromatic hydroxy moieties. More specifically, the present invention involves a process for preparing aromatic polyesters wherein the aromatic hydroxy moieties of the aromatic polyester monomers are acetylated with isopropenyl acetate in the polymerization vessel followed by polymerization to the aromatic polyester.
2. Description of the Prior Art
Wholly aromatic polyester resins, with the aromatic polyester being considered to be "wholly" aromatic in the sense that each moiety present contributes at least one aromatic ring to the polymer backbone, have long been known. For instance, 4-hydroxybenzoic acid homopolymer and copolymers have been provided in the past and are commercially available. Representative publications which discuss wholly aromatic polyester include: (a) "Polyesters of hydroxybenzoic Acids," by Russell Gilkey and John R. Caldewell, J. of Applied Polymer Sci., Vol. II, Pages 198 to 202 (1959), (b) "Polyarylates (Polyesters From Aromatic Dicarboxylic Acids and Bisphenols)," by G. Bier, Polymer, Vol. 15, Pages 527 to 535 (August 1974), (c) "Aromatic Polyester Plastics," by S. G. Cottis, Modern Plastics, Pages 62 to 63 (July 1975); and (d) "Poly(p-Oxybenzoyl Systems): Homopolymer for Coatings: Copolymers for Compression and Injection Molding," by Roger S. Storm and Steven G. Cottis, Coatings Plast. Preprint, Vol 34, No. 1, Pages 194 to 197 (April 1974). See Also, U.S. Pat. Nos. 3,039,994; 3,169,121; 3,321,437; 3,553,167; 3,637,595; 3,651,014; 3,723,338; 3,759,870; 3,767,621; 3,778,410; 3,787,370; 3,790,528; 3,829,406; 3,890,256; and 3,975,487.
Publications disclosing certain polyesters which exhibit melt anisotropy include (a) "Polyester X7G-A Self Reinforced Thermoplastic," by W. J. Jackson, Jr., H. F. Kuhfuss, and T. F. Gray, Jr. 30th Anniversary Technical Conference, 1975 Reinforced Plastics/Composites Institute. The Society of the Plastics Industry, Inc., Section 17-D, Pages 1 to 4, (b) Belgian Pat. Nos. 828,935 and 828,936, (c) Dutch Pat. No. 7505551, (d) West German Nos. 2520819, 2520820, 2722120, 2834535, 2834536 and 2834537, (e) Japanese Nos. 43-223; 2132-116; and 3021-293, (f) U.S. Pat. Nos. 3,991,013; 3,991,014; 4,057,597; 4,066,620; 4,067,852; 4,075,262; 4,083,829; 4,118,372; 4,130,545; 4,130,702; 4,156,070; 4,159,365; 4,169,933; 4,181,792; 4,183,895, 4,188,476; 4,189,996; 4,201,856; 4,224,433; 4,226,970; 4,230,817; 4,232,143; 4,232,144; 4,238,598; 4,238,599; 4,238,600; 4,242,496; 4,245,082; 4,245,084; 4,247,514; 4,267,304; and 4,269,965; and (g) U.K. Application No. 2,002,404.
Additionally, commonly assigned U.S. Pat. No. 4,161,470 claims a polyester of 6-hydroxy-2-naphthoic acid and para-hydroxy benzoic acid, and in commonly assigned U.S. Pat. No. 4,219,461 is claimed a polyester of 6-hydroxy-2-naphthoic acid, para-hydroxy benzoic acid, aromatic diol, and aromatic acid. Moreover, commonly assigned U.S. Pat. No. 4,256,624, claims a polyester of 6-hydroxy-2-naphthoic acid, an aromatic diol and an aromatic diacid. Each of these polyesters exhibits an anisotropic melt phase and is capable of readily undergoing melt processing to form quality fibers, molded articles, etc.
Several procedures are known for preparing aromatic polyester of fiber-forming molecular weight, including the Du Pont direct polymerization procedure. One commonly employed procedure involves thermally reacting aromatic acetate moieties with aromatic carboxylic acid moieties. In other words, the aromatic hydroxy groups are acetylated. The use of acetylated reactants has been of utmost importance when preparing a polyester of an aromatic hydroxy acid since such compounds tend to readily decarboxylate. For example, hydroxybenzoic acid is notorious for severe decarboxylation upon attempted direct esterification.
Several effective acetylating agents are known, among which is isopropenyl acetate, e.g., see "Reactions of Isopropenyl Acetate" by H. J. Hagemeyer, Jr. and D. C. Hull, Ind. and Eng. Chem., Vol. 41., No. 12, pp. 2920-2924 (1949); and, "Acylation. Part III. Acid-catalysed Acetylation by Isopropenyl Acetate" by E. A. Jeffery and D. P. N. Satchell, J. Chem. Soc., p. 1876 (1962). Propanoic acid anhydrides and other alkyl acid anhydrides are also mentioned in the literature as agents for the esterification of hydroxyl groups. In the esterification of aromatic hydroxy polyester monomers prior to polymerization, however, acetic anhydride has been the agent used foremost.
Generally, when employing acetic anhydride or an acetylation reagent, the aromatic hydroxy moieties are initially acetylated in a vessel other than the polymerization vessel. Upon completion of the reaction, the acetylated hydroxy monomers are separated from the reaction mixture and then transferred to the polymerization vessel for polymerization between the acetate moieties and aromatic carboxylic acid moieties. The separate acetylation reaction followed by the separation and transfer of the acetylated hydroxy monomers has been found necessary due to the deleterious effects resulting from conducting the polymerization in the acetylation reaction mixture. However, the operation of separating the acetylated aromatic hydroxy monomers and then transferring same to the polymerization vessel is time consuming and economically wasteful.
Processes have been disclosed wherein in situ acetylation with acetic anhydride has been conducted in the polymerization vessel. See, for example, U.S. Pat. Nos. 3,637,595 and 3,975,487 issued to Cottis et al wherein hydroxy benzoic acid, an aromatic diacid and aromatic diol are refluxed for an extended period of time at elevated temperatures in the presence of acetic anhydride. The quality of the resultant polymer, however, is generally less than that desired with respect to the polymer's ability to provide high strength and high modulus fibers.
The need therefore, has been ever present for a process of preparing wholly aromatic polyesters having a satisfactory inherent viscosity and ability to produce fibers of high strength and high modulus without the time-consuming and economic disadvantages of first having to separate acetylated monomers and then transfer them to a polymerization vessel.
Accordingly, it is an object of the present invention to provide a novel, effective and economically advantageous process for preparing polyesters involving the esterification, and in particular, acetylation, of aromatic hydroxy moieties.
It is another object of the present invention to provide an economic yet effective process for preparing wholly aromatic polyesters when using esterified, and in particular acetylated reactants, wherein but a single reaction vessel may be employed.
Still another object of the present invention is to provide a process for preparing wholly aromatic polyesters of suitable inherent viscosity to provide relatively high strength and high modulus fibers wherein the polymerization reaction can be effectively conducted in the reaction mixture resulting from the initial acetylation of aromatic hydroxy moieties. In other words, it is an object of the present invention to provide an economically advantageous process for preparing polyesters involving an essentially in situ acetylation of aromatic hydroxy moieties, yet which is still most effective in producing a high quality polyester capable of producing high quality fibers.
These and other objects, as well as the scope, nature and utilization of the invention, will be apparent to those skilled in the art from the following description and the appended claims.